The invention provides a multi-stage process for the preparation of a low-sulfur reformate from a mixture of hydrocarbons for use in a fuel cell system.
In general, fuel cells that produce electrical energy operate with hydrogen as the fuel. In motor vehicles, fuel cells produce the hydrogen from hydrocarbons using a hydrogen production system.
To produce hydrogen, it is known in the art that hydrocarbons can be reacted at high temperatures on a suitable catalyst in the presence of water vapor to give hydrogen, carbon monoxide and carbon dioxide. The reaction is highly endothermic and proceeds, for example, in accordance with the following chemical reaction:C8H18+8 H2O→8 CO+17 H2 ΔH=+1250 kJ/mol  (1)
The steam to carbon ratio, S/C, is a characteristic of this reaction. In chemical reaction (1) S/C is 1. The conversion of a hydrocarbon-containing gas mixture into a hydrogen-rich product gas mixture is called reforming. The resulting product gas mixture is the so-called reformate or reformate gas.
It is also known in the art that another way of producing hydrogen is by catalytic partial oxidation CPO. In this case, the hydrocarbons are reacted to give carbon monoxide and hydrogen in the presence of oxygen on a catalyst, for example in accordance with chemical reaction (2). An important characteristic of partial oxidation is the air to fuel ratio λ, which is defined as the ratio of the number of moles of oxygen used to the number of moles of oxygen required for complete oxidation (see chemical equation (3)):C8H18+4 O2→8 CO(g)+9 H2 λ=0.32 ΔH=−685 kJ/mol  (2)C8H18+12.5 O2→8 CO2+9 H2O λ=1 ΔH=−5102 kJ/mol  (3)
A third way for obtaining hydrogen is the so-called autothermal steam reforming, in which exothermic catalytic partial oxidation is combined with endothermic steam reforming. The reaction gas mixture for autothermal steam reforming is characterized by the air/fuel ratio λ as well as by the steam/carbon ratio.
For extensive conversion of the carbon monoxide formed during reforming, one or more process steps are provided downstream of the reforming step, in which carbon monoxide is reacted with steam to give carbon dioxide and hydrogen in an exothermic shift reaction in accordance with equation (4).CO+H2O⇄H2+CO2 ΔH<0  (4)
The residual carbon monoxide content of the reformate corresponds to the equilibrium concentration at the temperature of the outlet from the shift reactor. In order to reduce the carbon monoxide content of the reformate to less than 1 vol. %, the shift reaction is therefore often divided between two steps, a high temperature shift step and a low temperature shift step. As an alternative to the low temperature shift reaction, so-called gas separation membranes based on palladium alloys may also be used.
A process for operating a methanol reforming unit in which, during the reforming process, methanol is reformed in a methanol reforming reactor using a methanol reforming catalyst is known in the art. The problem with this process is the deactivation of methanol reforming catalysts based on Cu/ZnO at high loads. To regenerate the catalyst, the reforming process is interrupted periodically by catalyst reactivation phases. In the regeneration phases, the methanol reforming catalyst is used at a reduced load and/or a higher temperature than under normal operation or is rinsed with an inert gas.
Also, known in the art is a process for regenerating reforming catalysts in which regeneration is achieved by varying the amount of steam, air or fuel supplied during continuous operation. External heating of the reactor while keeping the reactant streams the same and also the addition of additives are described as other possibilities for regeneration.
It is desirable to use conventional engine fuels for the production of hydrogen by reforming hydrocarbons. These hydrocarbons, which are obtained from natural sources, always also contain relatively high concentrations of sulfur compounds. Diesel fuel typically contains between 100 and 1000 ppm of sulfur. The sulfur content of petrol is usually less than 100 ppm. Particularly low-sulfur petrol has a sulfur content of less than 10 ppm.
The presence of sulfur in the hydrocarbons reduces the catalytic activity of the catalysts for hydrogen production and also the activity of the anode catalysts in fuel cells.
In general, the process for preparing a hydrogen-rich reformate from hydrocarbons or mixtures of hydrocarbons is known in the art. Such a process usually consists of at least two process steps, wherein in the first step the reformate gas is obtained by catalytic steam reforming (STR) of a reaction gas mixture which contains sulfur-containing hydrocarbons and steam and is characterized by its steam/carbon ratio S/C. The carbon monoxide content of the reformate gas formed in this way is reduced in a second and subsequent steps sufficiently for the catalytic activity of the anode catalyst to be no longer substantially impaired by carbon monoxide. A carbon monoxide content of less than 100, preferably less than 50 ppm is sufficient for this purpose.
The process is particularly suitable for the reforming of hydrocarbons from natural sources, which always have a certain concentration of sulfur compounds. These sulfur compounds are reduced substantially to hydrogen sulfide due to the reductive conditions prevailing during reforming. The reformate leaving the reforming step, however, may still contain traces of organic sulfur compounds which have not been converted to hydrogen sulfide.
Some of the sulfur compounds are absorbed by the reforming and shift catalysts. The sulfur content of the reformate after leaving the catalytic steps, however, is still too high for the subsequent components.
The anode catalysts in the fuel cells are especially endangered, but any low temperature shift catalysts and gas separation membranes used to reduce the concentration of carbon monoxide are also at risk. Whereas the catalysts in the reforming steps and the high temperature shift step can be desulfurised by thermal treatment at temperatures above 600° C., which process largely regenerates the catalytic activity. This is not possible in the case of the anode catalysts, low temperature shift catalysts or any optionally used gas separation membranes. Conventional polymer electrolyte fuel cells are ruined at temperatures above about 140° C. and thus would not survive regeneration. Thus, poisoning of the anode catalysts by sulfur compounds in the reformate is irreversible.
The catalysts in the low temperature shift steps are also irreversibly poisoned by the sulfur components in the reformate gas because either they cannot be heated to the temperature required for desulfurisation, due to the way the process for the production of hydrogen is arranged, or they are not sufficiently heat resistant. Gas separation membranes, based on palladium, also frequently used to remove carbon monoxide from the reformate also have to be protected from sulfur compounds. Their permeation capacity is irreversibly damaged by sulfur compounds.
Based on the forgoing there is a need in the art for a process for preparing a low-sulfur reformate gas, the residual sulfur content of which is so low that poisoning of the anode catalysts is largely avoided. In addition, the process should enable regeneration of the reforming and shift catalysts under continuous operation.